For high temperature above 500.degree. C., three types of temperature measuring devices are commonly used. They are thermocouples, optical pyrometers and blackbody temperature sensors.
Thermocouple devices are generally inexpensive, but are not suitable for protracted use in harsh environments in which rapid temperature fluctuations are to be measured. If thermocouples are made very small to reduce their thermal mass and thereby to improve their high frequency response, they then become too fragile for many applications. Furthermore, they cannot withstand prolonged exposure to very high temperature without degradation and the need for periodic replacement. First, the thermocouple junction may change composition or become contaminated, upon prolonged exposure to high temperature resulting in inaccuracies due to a change in calibration. Second, a form of hysteresis may be caused upon repeated temperature cycling by temperature induced changes in grain size of the materials forming the junction.
Radiation pyrometers are more suitable for measuring very high temperatures in that the sensor need not be located in the high temperature environment. However, specific knowledge of emissivity, which is difficult to obtain in practice, must be available for an accurate conversion of the intensity of the measured radiation into temperature. Furthermore, the environment to be measured must be visible by an non-absorbing light path from the detector location, so that the radiation emitted can be measured correctly. Reflections and radiation from region other than of interest can also pose serious complications if these contribute significantly to the radiation received by the detector.
Blackbody or greybody temperature sensors are increasingly becoming preferred as high temperature measuring devices because they do not suffer from the disadvantages discussed above. A blackbody temperature sensor is based on the principle of detecting the radiation emitted from a blackbody emitter placed in a high temperature environment the temperature of which is to be measured. A theoretical blackbody has the property of zero reflectivity, and a emissivity of unity. In practice, when the emissivity is less than unity but is substantially independent of wavelength, the blackbody emitter is sometime referred to as a "greybody" emitter. When the blackbody emitter is at thermal equilibrium with the environment, the radiation emitted by the blackbody is a known function of the temperature of the environment, as given by well-defined physical laws. Furthermore, the calibration of the blackbody sensor can be determined in advance of the measurement. Thus, by detecting the radiation from the blackbody emitter, the temperature of the environment can be accurately determined. The radiation is usually guided to an external detector and instrumentation for sensing and displaying the temperature. Typically, the blackbody emitter is disposed at the tip of an optical lightpipe fabricated from a suitably transparent material able to withstand the temperature of interest. The radiation is guided through the optical lightpipe to a cooler region where it may be further relayed by conventional optical fiber to a remotely located detector. Blackbody measuring devices have found applications, among other things, in monitoring temperatures inside environments such as an internal combustion engine, a gas turbine, or a rocket exhaust stream. These environments are characterized by high temperature (500.degree.-2000.degree. C.), rapid temperature fluctuations (e.g. 5000 Hz) and high flow rates (as much as Mach 2). To adequately monitor the rapid temperature fluctuations, the blackbody emitter must have a fast response time with a frequency response in the thousands of Hertz range. In order to achieve this order of frequency response, prior art has focused on minimizing the effective thermal capacity of the blackbody emitter. Those blackbody cavities are typically formed by evaporating or sputtering a metallic film of a few micrometers in thickness onto the tip of a lightpipe. Maximum frequency response has been achieved by the low thermal mass and high thermal conductivity of the thin metallic film.
For example, U.S. Pat. No. 3,626,758 to Stewart et al., discloses a remote radiation temperature sensor for a gas turbine. A blackbody radiator is formed on the conical tip of a tubular body. The blackbody radiator is preferably thin and formed of a metallic material of high thermal conductivity. The metallic material may be formed by vacuum deposition of the predominantly nickel and molybdenum alloy known as Hastelloy.
U.S. Pat. No. 4,576,486 to Dils, discloses an optical fiber thermometer employing a blackbody cavity at a tip of a high temperature optical fiber. The blackbody cavity is formed by sputtering a thin optically dense metallic or oxide coating on the surface of the optical fiber.
Similarly, Chinese Patent, Application No. 89200371.5, Publication No. 2046210U to Zhou, et al. discloses a blackbody temperature sensor formed by sputtering a layer of high temperature, non-oxidizing material onto a lightguide.
U.S. Pat. No. 4,679,934 to Ganguly et al., discloses Fiber Optic Pyrometry with a Large Dynamic Range. A blackbody radiating member is preferably fabricated as an iridium metal film on an optical fiber, while a protective film layer is preferably fabricated as an aluminum oxide film.
While a blackbody cavity formed by a sputtered thin metallic film has fast response, it is nevertheless not very durable. The metallic film will generally not last very long in abrasive environments with high flow rates such as inside an internal combustion engine or a gas turbine. In fact, a blackbody temperature sensor with a platinum film may only last over the duration of one temperature measurement in a gas turbine experiment. It should be noted that even an accidental scratch on the film can alter its effectiveness as a blackbody and hence its accuracy as a sensor.
Attempts have been made to improved the durability of the thin metallic film by adding a protective overcoat to it. For example, U.S. Pat. No. 4,576,486 to Dils also discloses an overcoating protective film. The protective film, 1 to 20 micrometers thick, is formed by sputtering aluminum oxide over the metal film which is located on a sapphire or zirconia fiber.
Another disclosure is found in Hypszer et al., "Optical Fibre Temperature Sensor Based on a BlackBody Radiation", SPIE Vol 1085, Optical Fibres and Their Applications V, pp. 476-479, (1989). This article discloses forming a blackbody on a quartz rod. The blackbody consists of a metallic layer surround a tip of the quartz rod. The metallic layer is formed by evaporation of chromium. A protective layer of silicon monoxide covers the metallic layer.
In practice, for high temperature blackbody sensors, protective layers have typically been applied as a sputtered, thin polycrystalline oxide film not exceeding a few micrometers thick. Protective layers more than a few micrometers have not been used for several reasons. First, considerable time and cost are required to sputter a thick layer of oxide material. Secondly, a thicker layer would appear to decrease the effective thermal responsivity of the thin film blackbody, thereby compromising the response time. Thirdly, and more seriously, a thick protective overcoat layer takes on its own bulk material characteristics and may not reliably adhere to the lightpipe or substrate. Having the metallic thin film sandwiched between the oxide materials makes the situation worse. This is because the device is subjected to a very wide temperature range from room temperature to thousands of degrees Celsius. Over this range, the materials forming the substrate and the protective overcoat would not only change their relative dimensions dramatically but may also have undergone phase changes. The differential volume changes and surface properties over the temperature range would readily crack or dislodge a thicker and poorly adhering protective overcoat.
A thick ceramic coating (typically 1 to several millimeters thick) forming a blackbody temperature sensor has been disclosed by Dr. Zhihai Wang in "Blackbody Optical Fiber Thermometers and their Applications, " Ph.D Thesis, Qing-Hua University, Beijing, Peoples Republic of China, 1989. The coating is formed by baking a layer of metal oxides mixture onto a sapphire substrate. The metal oxides mixture is essentially zirconia stabilized by CaO, MgO and Y.sub.2 O.sub.3. The technique is to bake the layer for an extended period (e.g. 12 hours) at a temperature (e.g. 1000.degree. C.) well below the melting point of the ingredients in the mixture. This results in the formation of a highly porous ceramic layer. The incorporation of many air spaces into the layer is essential for absorbing some of the differential volume change described earlier and prevents the thick coating from cracking or dislodging from the substrate. However, this device has poor frequency response and is time-consuming to make. The ceramic layer is relatively thick in order to build up sufficient opacity to form a blackbody cavity. The large bulk and the poor thermal conductivity of trapped air result in a slower responding device.
On the other hand, a thin protective layer of a few micrometers formed by conventional sputtering techniques offers little protection to abrasion in a high fluid flow rate environment. Nevertheless, despite the high cost of sputtering an oxide or ceramic material, a protective layer is applied to some application-specific temperature sensors to isolate the underlying platinum film from possible catalytic interactions with the high temperature environment, and generally to provide additional mechanical protection of the platinum film.
Other solutions have been attempted by practitioners in the field, such as in the following disclosures.
U.S. Pat. No. 4,794,619 to Tregay, discloses an Optical Fiber Temperature Sensor where the blackbody sensor is formed by a cavity inside the optical fiber.
U.S. Pat. No. 4,906,106 to Kaufman et al., discloses a pyrometric temperature measuring instrument in which a blackbody sensor is formed by first removing the cladding at a tip of a cladded glass fiber and applying to the nude tip a black paste consisting of a mixture of finely dispersed carbon and silicon. This technique is only applicable for low temperature measurements, as the matrix will decompose in air at a few hundred degrees C.
European Patent Application No. 90311575.6, Publication No. 0 425 229 A1 to Lee et al., (Priority U.S. patent application Ser. No. 07/427,179, filed Oct. 23, 1990) discloses a hollow light guide being used to transmit radiation from an emissive member to a detector. The hollow light guide is made of high temperature metals, ceramics, metal alloys, materials with dielectric coating, or ceramic materials with metallic film coating. The emissive member may be one of three forms: a dot; a film or coating; or a blackbody cavity. For a film or coating, it may be applied by sputtering, evaporation, dipping, coating, etc. to close an opening at a tip of the hollow light guide. For a blackbody cavity, it is a separate structure mechanically mounted at a tip of the hollow light guide. The blackbody cavity may be made of high temperature metal, but can be also made of a transparent ceramic sphere or cavity with its surface sputtered with an optically dense film of metallic or oxide coating. It is apparent that this system would not have good high frequency response.
Holmes, "Fiber Optic Probe for Thermal Profiling of Liquids During Crystal Growth", Rev. Sci. Instrum., 50(5). pp. 662-663 (May, 1979), discloses a temperature probe formed by a small bead of graphite cement on a quartz optical fiber. The article states that the response time is 10 times faster than a conventional sheathed thermocouple due to the absence of a convention protective sheath.
However, these attempts provide only partial solutions to the above-mentioned problems while creating new problems and compromises of their own.
There is still a need for a rugged and low cost temperature measuring device which is suitable for applications in high temperature environments including that characterized by rapid temperature fluctuations and high flow rates.